Reduction of etoposide induced cell killing by hyperthermia can occur without changes in etoposide transport or DNA topoisomerase II activity

Abstract

We investigated the modification of etoposide (i.e. VP-16)-induced cell killing by hyperthermia in a radioresistant human melanoma (Sk-Mel-3) and a human normal (AG1522) cell line. VP-16, a DNA topo II poison, was given as a 1 h exposure at variable doses up to 35 μM; hyperthermia was given either before or following VP-16 treatment. Hyperthermic treatment comprised one of the following: 41°C for 8 h, 42°C for 2h or 45°C for 15 min. Hyperthermia preceding VP-16 treatment reduced the cytotoxicity of the latter; the reduction of VP-16 cytotoxicity was directly proportional to the severity of the hyperthermic treatment. For a particular combination of hyperthermic dose and VP-16 concentration, generally similar responses were seen in both cell lines. There were no effects on VP-16 cytotoxicity when both Sk-Mel-3 and AG1522 cells were heated at 41°C for 8h following treatment with VP-16. However, heating both cell lines at 45°C for 15 min following VP-16 treatment again reduced the amount of cytotoxicity associated with VP-16. In addition, we found that a preceding exposure to 45°C, 15 min heating did not affect either cellular accumulation or efflux of [³H]VP-16 in both cell lines. This suggested that the reduction in VP-16 cytotoxicity observed under those conditions was not due to a modification of VP-16 transport. We found no differences between the catalytic activities of topo II extracted from nuclei of Sk-Mel-3 and AG1522 cells that were either heated at 45°C for 15 min or that were not subjected to such treatment. These results therefore suggested that the substantial reduction of cytotoxicity seen when 45°C, 15 min heating preceded VP-16 treatment was also not due to an effect on topo II catalytic activity. Our results therefore demonstrate that hyperthermia, given either before or after VP-16, can actually reduce the amount of VP-16 cytotoxicity and that this can occur without any overt changes in VP-16 accumulation and efflux or in topo II catalytic activity.

Key Words:

• Etoposide
• hyperthermia
• topoisomerase activity
• drug transport